(Prior Art Statement)
Representative of the closest known prior art is IBM Technical Disclosure Bulletin, Vol. 13, No. 9, pages 2792-93 (February 1971), entitled "Generation of Double Size Characters," by C. J. Holderness; U.S. Pat. No. 4,129,860 to S. Yonezawa, filed Sept. 13, 1976 (priority Japan Sept. 12, 1975), issued Dec. 12, 1978, entitled "Apparatus for Forming a Character by a Matrix Pattern of Picture Elements;" U.S. Pat. No. 4,107,664 to F. C. Marino, filed July 6, 1976, issued Aug. 15, 1978, entitled "Raster Scanned Display System;" IBM Technical Disclosure Bulletin, Vol. 21, No. 11, pages 4339-40 (April 1979), entitled "Text Editing Display System with Vertical Expansion of Selected Lines for Highlighting," by C. E. Boyd, K. R. Demke, and J. L. Mumola; and U.S. Pat. No. 4,168,489 to J. L. Ervin, filed Feb. 13, 1978, issued Sept. 18, 1979, entitled "Full Page Mode System for Certain Word Processing Devices."
One of the ongoing trends in cathode ray tube (CRT) text processing system displays has been an effort to increase the amount of text capable of being displayed at once on the display screen. This has been done in either or both of two ways. That is, a larger CRT can be used or the physical size of the characters can be decreased to allow more characters to be displayed in a given frame size. The utilization of larger tubes entails higher costs and power requirements for the display system. This is because of greater costs associated not only with the larger CRT but also with higher performance deflection circuitry to deflect the CRT beam longer distances within the larger CRT screen area at the same refresh rate. This higher performance circuitry also requires more electrical input power. On the other hand, if the size of the CRT is held constant and the physical sizes of the characters and symbols to be displayed are made smaller in an effort to display more characters per frame, the difficulty of comfortably reading the characters increases.
In some modes of operation such as initial key entry of text, it is considered acceptable from an operator standpoint to utilize characters having a rather small physical size, since the operator in this mode normally does not do a significant amount of reading from the screen. However, in proofreading or editing and correction applications, it is desirable to display characters of a much larger physical size even if this renders the display of a full page of text impossible.
In the above-referenced IBM Technical Disclosure Bulletin article by Boyd, Demke and Mumola it is taught to display characters in a font having twice the vertical dimension of the character size normally displayed, without any expansion in the horizontal direction. If this technique were used for all of the text in a frame it would, of course, halve the amount of text that could be displayed in the frame. A variation of this technique that has been proposed would employ the use of the shorter characters on most of the lines of a frame and the larger characters on the line including the cursor character as well as, perhaps, one or two lines above and below the line including the cursor character.
Although the above referenced IBM Technical Disclosure Bulletin shows a system with double high characters, the project development which lead to that publication used separate character generator memories for the short character font and the tall character font. Since it was necessary in the case of each double high character to store twice as many bits of video data representative of the pels of the character as was required for the shorter characters, the use of that technique lead to the total requirement of three times as many bits of character generator memory as was originally required for the shorter, single high character font.
The above-referenced IBM Technical Disclosure Bulletin article by Holderness teaches an example of using the same character generator for generation of two sizes of characters. The dimensions of the displayed characters are doubled in both the horizontal and vertical directions. U.S. Pat. Nos. 4,107,664 and 4,168,489 also teach other examples of expansion of the character patterns in both the horizontal and vertical directions from a single set of characters stored in a single character generator memory.
It is usually the case that when it is attempted to expand the size of a set of characters represented by a single character font for use in more than one size, the appearance of many of the characters becomes undesirable because of differences in the proportions of the characters after enlargement. The change in proportions occurs because the character is expanded only in the vertical axis and not in the horizontal axis. Since the aforementioned patents and Holderness publication expand in both axes, they do not suffer from this problem. But, when it is attempted to construct a double high character by simply providing pairs of vertically disposed pels for each single pel of the single high character, the proportions of the various segments of the character may have an unusual and unacceptable appearance. This makes reading difficult and increases the operator's error rate. Of course, one way to overcome this problem is to separately store two different character fonts in three times the amount of memory space as is required for the single high font and aesthetically style both fonts differently from each other to achieve a desirable appearance in both sizes. The cost disadvantage of this approach is immediately obvious, however, when consideration is given toward tripling the character generator memory size.
U.S. Pat. No. 4,129,860 addresses the problem of enlarging characters stored in a single character generator while maintaining a clear and pleasant appearance of the character. The solution proposed by this patent, however, involves a real time interpolation technique including a substantially extensive amount of hardware. Further, this solution involves expansion of the characters in both the vertical and horizontal axes. While this may be an appropriate solution for the general case in which a widely varying degree of magnification of the characters and symbols is required, it appears to be an expensive approach for a system requiring a small number, for example a pair, of character sizes.
Except for the Boyd, Demke, and Mumola publication, the other examples of prior art teach character expansion in the horizontal as well as vertical axes. This potentially reduces the number of characters that can be displayed on a given screen size by about twice the amount as that experienced when the characters are expanded in only the vertical axis. However, the technique selected by Boyd, Demke, and Mumola in expanding in only the vertical axis required two character generaters to achieve adequantely styled characters in both sizes.
It would therefore be highly desirable to provide a very simple technique to enable the display of single and double high characters having a pleasant appearance in both sizes with a single character generator and a minimum requirement of associated character size translation hardware to enable the generation of two different character sizes from the single character generator memory.